Anthracite combustion characteristics and NO_xformation of a 300 MW_edown-fired boiler with swirl burners at different loads after the implementation of a new combustion system

A new combustion system has been applied to a 300 MW_edown-fired boiler with swirl burners to reduce NO_xemissions. The unit provided the introduction of overfire air (OFA) and a decrease in the flow area of the inner and outer secondary air ducts of the swirl burners. Industrial experiments on the retrofitted boiler were performed at different loads. Full-scale measurements of the flue gas temperature distribution in the burner outlet region, the furnace temperature distributions measured by a pyrometer and the local mean gas species concentrations in the region near the sidewall were made at loads of 180, 250, and 300 MW_e. The results show that the ignition distance increased with decreasing load, especially as the load decreased from 250 MW_eto 180 MW_e. At three different loads, the retrofitted coal/air flow could be all ignited in time at a distance in the range of 0.6–1.4 m from the burner outlet. Compared with the original combustion system, the ignition distance of the coal/air flow was significantly reduced at a load of 300 MW_e. In addition, at a load of 300 MW_e, the temperature of the boiler hopper was much higher than that at loads of 180 MW_eand 250 MW_e. Compared with the original combustion system, the upper furnace temperature decreased slowly with increasing measurement height at a load of 300 MW_eafter the retrofit. Measurements of the O₂and CO concentrations in the region near the sidewall indicate that the fullness degree of the coal flame in the furnace at different loads was different. After the retrofit, the reheat steam temperatures reached the design temperature of 541 °C at loads of 180 MW_eand 250 MW_e, and the average reheat steam temperatures increased by approximately 13 °C. Compared with the original combustion system, a significant NO_xreduction (more than 40%) at different loads was achieved without increasing the levels of unburnt carbon in the fly ash.